You can find this answer in book 2, chapter 9. An excerpt of Goldstein's book is provided. Within that excert, chapter 3 in entitled "War is Peace". It establishes the reasoning for maintaining wartime as opposed to peace.
For each of the superpowers, they have come to the realization that war achieves several means. First and most important, it uses up the surplus of goods produced by the people. This helps maintain a controlled society because if some people began to use more goods than others or rationing was done away with, wealth would occur for some. This would certainly disrupt the order. Without a lower class, Big Brother and the other government hierarchies have no one who needs and depends on them therefore remaining subject to them.
The basis of war is acceptable to people. It is something they have lived with. To the governments, it gives the people a reason to fear, they fear something else, never their own government. It also helps them place their hate. People will hate. This is a way to control where that goes.
As was mentioned in the previous post, continuous warfare is necessary for each of the superpowers to destroy the surplus of production. Before the atomic war, citizens lived in a society where they had the ability to gain wealth. With wealth came new opportunities to learn, advance, and realize that there is no need for an elite class. The problem became how to keep the wheels of production turning without increasing the wealth and well-being of the population. The solution was constant warfare. During war, citizens freely give power to the elite class to ensure their security while necessary destruction is accomplished. Continuous warfare also keeps the citizens of the superpowers in constant psychological distress. Through an atmosphere of hysteria, the population is easily controlled. War keeps the citizens in a frightened, hysteric emotional state while simultaneously increasing the power and influence of the elite. Destruction of surplus also creates a stagnant economy where social mobility and advancement is impossible. Continuous warfare is preferred to peace because the majority of the population cannot usurp power from the elite by developing and becoming intelligent. Warfare suppresses the population. In contrast, peace helps lower-class citizens advance in society. In order for the elite to stay in power, the masses must be oppressed and manipulated.